1. Field of the Invention
This disclosure relates to ion-exchange membranes. More particularly, this disclosure relates to resilient and chemically stable ion exchange membranes prepared by polymerizing ionic surfactant monomers onto and within a porous matrix.
2. Description of Related Art
Ion exchange membranes are used in electrodialysis, electrolysis, and diffusion dialysis where selective transport of ions takes place under the influence of an ion concentration gradient or an electrical potential gradient as the driving force. Historically, ion exchange membranes have been prepared by the copolymerization of a crosslinked divinyl monomer, such as divinylbenzene or ethylene glycol dimethacrylate, with monomer-containing ion exchange groups exemplified by 2-acrylamido-2-methylpropane sulfonic acid and by monomers that can be modified after polymerization with ion exchange groups exemplified by styrene and dimethylaminopropylacrylamide.
U.S. Pat. No. 3,451,951 discloses a multi-step process for preparing ion exchange membranes using the copolymers of styrene and divinylbenzene to provide good electrochemical properties and satisfactory mechanical properties. However, the multi-step process involves the use of hazardous chemicals such as styrene, divinylbenzene, concentrated sulfuric acid, and halogenated chemicals thereby causing the manufacturing process to be costly with significant chemical disposal problems.
U.S. Pat. Nos. 4,231,855, 4,587,269 and 5,264,125 disclose one-step processes for production of ion exchange membranes directly from monomers containing ionic functional groups. The final ion exchange membranes require no further chemical reactions after polymerization. Anionic monomers for cation exchange membranes include sodium 4-vinylbenzenesulfonate, 3-sulfopropyl acrylate potassium salt, and 2-acrylamido-2-methyl-1-propanesulfonic acid. Cationic monomers for anion exchange membranes include 3-acrylamidopropyl trimethylammonium chloride, 2-acryloyloxyethyl trimethylammonium chloride, 2-methacryloyloxyethyl trimethylammonium chloride, 3-methacryloylaminopropyl trimethylammonium chloride, and vinylbenzyl trimethylammonium chloride. However, a large percentage of solvents (>40 wt % of total monomer solution) in the formula have to be used in the one-step process due to technical challenges associated with the incompatibilities between these highly polar ionic monomers and water-insoluble non-polar crosslinking monomers. The ionic monomers and hydrophobic crosslinking monomers cannot be blended together because of their large differences in polarity. High percentages of solvent content in the formulae lead to final ion exchange membranes having high-water contents and poor ion-selective permeabilities. In addition, in order to restrain the osmostic swelling of hydrophilic and ionic components of ion exchange membranes, large amounts of cross-linking monomers (>50 mol % of the total monomer contents) have to be used in the membrane formulae, making the final membranes brittle in nature. Ion exchange membranes from the one-step process generally have poor mechanical properties.